Noble gas isotopes that can be nuclear spin-polarized are very attractive imaging agents because of their high detection sensitivity for nuclear magnetic resonance. In particular, spin-polarized 129Xe enables fast, gas- space imaging of the lungs and airways for clinical diagnosis and physiological studies. Unfortunately, the high-cost of 129Xe limits the number of high-resolution lung imaging studies that can be performed. We propose to develop a system to recover and recycle isotopically enriched xenon gas so that high-resolution lung imaging can become common and inexpensive. The innovation is a cryogenic gas separation system that extracts and purifies the 129Xe from a patient's exhalation stream and saves it for re- polarization and reuse in other patients. In Phase I, we will prove the feasibility of the 129Xe recovery system by: (1) performing a proof-of- concept test that demonstrates the feasibility of efficiently separating, purifying, and sterilizing the 129Xe from exhalation gases, and (2) refining a conceptual design of a complete system to recover and recycle 129Xe. In Phase II, we will build and demonstrate a complete recovery and recycling system. PROPOSED COMMERCIAL APPLICATIONS: Rare, noble gas isotopes are a potential breakthrough for medical imaging if they could be used economically. Uses for these isotopes include high- resolution, gas-space imaging of the lungs and airways, practical low-field MRI, studies of blood perfusion and brain physiology, and porous media studies based on diffusion of gaseous isotopes. Technology developed in this program will make these advances practical and economical by enabling these rare isotopes to be reused many times.